Method of making a radial bearing containing end-loaded filaments

ABSTRACT

The invention is directed to a method of making a radial bearing wherein filaments are wound diametrically across turns of an expanded helix. When each turn of the helix is completely covered with filaments, the helix is compressed and compacted. A shaft passage is then formed through the center of the helix to form the bearing.

States Patent Unite Stiff et a1.

[ 1 Mar. 27, 1973 METHOD OF MAKING A RADIAL BEARING CONTAININGEND-LOADED FILAMENTS Inventors: Bernard G. Stifi, Lynnfield; Thomas M.Finelli, North Andover, both of Mass.

Assignee: Avco Corporation, Cincinnati, Ohio Filed: Feb. 14, 1972 Appl.No.: 226,344

Related US. Application Data Division of Ser. No. 65,646, Aug. 20, 1970,Pat. No. 3,675,980.

US. Cl ..29/14 9.5 NM, 29/ 149.5 PM, 29/419,

' 308/238 Int. Cl. ..B2ld 53/10, B23p 17/00 Field of Search ..29/149.5PM, 149.5 NM,

29/1495 R, 149.5 DP,420,420.5,419; 308/238 [56] References Cited UNITEDSTATES PATENTS 2,560,134 7/ 1951 Schroeter ..29/149.5 NM X 2,607,9828/1952 Hack et al ..29/149.5 PM X Primary Examiner-Thomas H. EagerAttorney-Charles M. Hogan et a1.

[57] ABSTRACT The invention is directed to a method of making a radialbearing wherein filaments are wound diametrically across turns of anexpanded helix. When each turn of the helix is completely covered withfilaments, the helix is compressed and compacted. A shaft passage isthen formed through the center of the helix to form the bearing.

1 Claim, 5 Drawing Figures PATENTEUHARZYISYS 722,051

r 1. I LOVAD 4 Q 2o 5 METHOD OF MAKING A RADIAL BEARING CONTAININGEND-LOADED FILAMENTS This invention is a division of the parent caseentitled A Bearing," Ser. No. 65,646, filed Aug. 20, 1970, now US. Pat.No. 3,675,980.

Heretofore, bearings have been constructed out of block carbon-graphitematerials. Bearings have also been constructed of carbon-graphitefilament composites. Heretofore, some of these composites have beenformed so that the filaments are end loaded. However, on composites thegraphite, at most, formed up to 50 or 55 percent of the compositebearing surface.

It is an object of the invention to provide a method of making a bearingconsisting of end-loaded filaments.

It is yet another object of the invention to provide a method of makingan end-loaded graphite filament bearing where the filament containsgraphite crystals with their slip planes oriented longitudinally.

It is yet another object of the invention to provide a method of makingan end-loaded carbonaceous filament bearing with an ability to handleexceptionally high pressure velocity product.

It is still another object of the invention to provide a method ofmaking an end-loaded carbonaceous filament bearing consisting ofcarbonaceous filaments forming a mass of at least 70 percent of thetheoretical density of the graphite fibers.

.It is still another object of the invention to provide a method ofmaking an end-loaded carbonaceous filament bearing which avoidsthedisadvantages and limitations of prior art carbon-graphite bearings.

It is still another object of the invention to provide a method ofmaking a bearing having anisotropic thermal expansion characteristicssuch that the bearing remains secured in its housing at hightemperatures.

It is still another object of the invention to provide a method ofmaking a porus carbonaceous bearing.

It is yet another object of the invention to provide a method of makingan end-loaded graphite filament bearing having oriented crystallites sothat the coefficient expansion of the graphite may be matched to thenon-carbon complementary members of the bearing such as housings andshafts.

It is a further object of the invention to provide a method of making agraphite bearing consisting essentially of graphite filaments whichincludes protective means against overheating.

The invention also covers a bearing having anisotropic thermal expansionproperties.

In accordance with the invention, a carbonaceous bearing consists ofcarbonaceous filaments arranged'in a side by side relationship to form amass which is at least 70 percent of the theoretical density of carbon,the bearing surface is defined by the terminal cross sectional surfacesof the said filaments.

The novel features that is considered characteristic of the inventionare set forth in the appended claims; the invention itself, however bothas to its organization and method of operation, together with additionalob- 5 jects and advantages thereof, will best be understood Also, inaccordance with the invention, a method of from the followingdescription of a specific embodiment when read in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic representation of a pair of graphite filamentswith crystallites aligned longitudinally;

FIG. 2 is a schematic representation of a thrust bearing containingaxially aligned carbonaceous filaments;

FIG. 3 is a schematic representation of a radial bearing with axiallyaligned carbonaceous filaments;

FIG. 4 is a schematic representation of an expanded right circular helixshowing diametrically disposed carbonaceous filaments; and

FIG. 5 is a schematic representation of the surface formed by one turnof the helix covered with diametrically disposed filaments.

Heretofore, carbon-graphite bearings were formed largely from blockmaterial. At the outset if should be made clear that carbon and graphiteare not the same. Carbon is an amorphous substance, showing very littleor no crystalline structure and, generally, is the precursor materialfor graphite. Graphite is a crystalline material formed by heatingcarbon to a very high graphitizing temperature. The term carbonaceous isa generic designation covering carbon and graphite, mixtures of carbonand graphite or other forms thereof. Unless one deliberately sets out toform a' graphitic material with crystals of a particular unidirectionalorientation, the crystals in commercially available graphite comerandomly oriented.

The preferred form of obtaining and using carbonaceous filaments forthis application is in a yarn or roving.

A number of important characteristics result from orienting the crystalsin graphite. The properties of the materials become anisotropic.

Broadly, the invention is directed to a bearing made up of end-loadedcarbonaceous filaments in a side by side relationship. The filaments arehighly compacted so that accumulation of the ends of these filamentsform a mass having a density which is at least percent of thetheoretical density of the filament material.

The'resulting structure is porous. There is adequate paths for forcingfluids, oils or gas for example, through the carbonaceous mass to thebearing surface. The impregnated mass provides effective cooling viatranspiration. Coolant used to cool may be continuously replaced throughthe carbonaceous mass from a remote reservoir.

The emphasis is placed on the filament ends since, as will be pointedout, there may be structures, such as the radial bearing, having regionsremote from the bearing surface having a density of less than 70percent. These so-called low density non-bearing regions may beimpregnated with a matrix material, such as a resin or other filaments,or remain porous.

End-loaded carbonaceous filaments in a 70 percent or greater densityconfiguration can handle higher PV (Pressure-Velocity product) valuesthan is attainable with conventional bearing material. For example,Table 1 provides comparative data of bearing materials tested undercontinuous service of 10,000 hours. The PV values listed resulted inequal wear for the bearing materials listed.

TABLE 1 Material PV-Value Bronze 25,000 Iron 25,000 Aluminum 25,000Bronze impregnated with graphite 30,000 Bulk carbon-graphite 15,000 Bulkcarbon-graphite impregnated with resin 12,000 Bulk carbon-graphiteimpregnated with boron IS,000 End-loaded carbon filaments 100,000

Referring to FIG. 1 where there is shown a pair of graphite filaments ina side by side relationship in a schematic bearing environment, eachfilai'nent contains a plurality of crystallites l 1 each of which ismade up of highly oriented carbon crystals. The crystallites 11 arediagrammatically shown elongated longitudinally to indicate asubstantially unidirectional orientation of the carbon crystals so thattheir slip planes are longitudinally oriented.

The filaments 10 are arranged so that they are endloaded by thesimulated motion. Wear occurs longitudinallyfThe heat generated at theinterface of a bearing surface 12 formed from the terminal crosssectional surface is conducted along the filaments to a heat sink 14which is typically the bearing housing. The spaces 16 between thecrystallites 11 in the filaments 10 are pores and/or elemental carbon,primarily carbon.

however.

Preferably the filaments 10 have a diameter of 0.0004 inches. The truecriteria for selecting a filament diameter rests in the users ability toachieve a minimum density of at least 70 percent as heretoforespecified. Generally, large diameter filaments have a poor stackingfactor. While smaller diameter filaments stack efficiently to providehigher density masses, extremely fine filaments are virtually impossibleto handle at the present time.

In FIG. 2 a thrust bearing is shown. The bearing is based on theprinciple of loading the ends of the filaments in accordance with thisinvention. The first bearing of the type that would normally be slippedonto a rotating shaft. Hence, it is an annulus in cross section. Atypical thrust bearing 15 is comprised ofthree concentric rings: anoutside tensile ring 18, an inside compression ring 19, and the bearingannulus 17 in the middle.

The filaments in the FIG. 2 thrust bearing are in a parallelrelationship forming the right circular cylinder 17. The cylinder 27 hastypically a uniform structure and, in accordance with this invention, adensity equal to at least 70 percent of the theoretical density of thecarbonaceous material used.

One technique for compacting the annulus 17 is to expand the diameter ofthe inside ring 19 until the filaments are pressed very closely togetheragainst the outer tensile ring 18. In the event the inside ring 19 hasbeen forced to exceed its own yield point, the entire assembly willremain fixed in the expanded position. In the alternative, the insidering 19 may be kept in compression by forcing a plug into the hole ofthe inside ring to maintain the stretched position. Such an assembly cannow be cut to length, ground, and made to be a very cheap method ofmanufacture.

A radial bearing 20 which also uses the end loading characteristics isshown in FIG. 3. In this design, radial elements are arrayedas spokes ina wheel. As in any wheel, the spokes become wider apart as they proceedoutwardly towards the rim of the wheel. The opposite is true, of course,as the spokes come in towards the wheel axial. In the radial bearing 20the spokes actually touch each other at the bearing surface. There is noinner hub included in the bearing, however, since the rotating shaftserves this purpose. The inner diameter of the bearing (where thefilament ends touch each other) is the surface that takes the radialload when the radial bearing is slipped onto the rotating shaft. Theends form a mass having a density of percent of the density of thefilaments.

One of the best known characteristics of a graphite crystal is its lowcoefficient of friction. This is best observed when the crystallinematerial is rubbed by another surface in the longitudinal directionparallel to the crystal planes. A single graphite crystal has shearcharacteristics similar to a pack of cards, and it deforms under load inmuch the same manner. The slip planes in the crystal can be made to runin the direction of the yarn. Such is the case in FIG. 1 where thepredominant crystal orientation is along the direction of the filament.

The coefficient of friction of the crystal in the lateral direction(crosswise to the plane of slip) is much higher, however. Very littleslippage of the planes takes place and the wear action becomes adifferent phenomenon. The values of the friction coefficient are asfollows: slip plane direction, 0.1; perpendicular to slip plane, 0.3.

The wear rate is, to a considerable degree, a direct indication of theremoval process. The wear process is different when parallel to thecrystal planes than when perpendicular to them. when the action isparallel to the crystal planes they slide easily on each other and theresulting wear rate in this direction is high. The wear rate normal tothese planes is much lower since there is very little plane slippage.The basal planes slip off endwise, and the shearing action does notappear to propagate. laterally from one crystal plane to another.Virtually no dust is apparent in such a wearing action.

One of the major causes of carbon-graphite'bearing design mistakes foroperation at elevated temperatures concerns the coefficient of expansionof the bearing material in relation to the bearing housing and theshaft. This problem is probably best exemplified in the article inProduct Engineering, Apr. 1964.

Summarizing briefly, the high operational temperatures, the metalhousing containing the carbon-graphite bearing material, and the shaftexpand at a faster rate than the carbonaceous material. Very often a gapwill occur between the carbon and its housing causing the carbon torotate within its housing. Should the shaft expand sufficiently to seizethe carbon bearing, there is a further tendency for the carbon to rotatewithin its housing. In the event such rotation does not occur, the shaftquickly destroys the surface of the carbon bearing.

The aforementioned problems are accounted for by conservative and costlydesigns, auxiliary cooling to prevent overheating, or design low leveloperational temperatures. All of the forgoing are wasteful andinefficient. These problems tend to be minimized and eliminated throughthe practice of the inventive concept described herein.

The bearing temperatures is a function of the rate and quantity of heatgeneration, the heat conductivity of the bearing, the adequacy of theheat sinks and the various heat transfer coefficients in the bearings asa whole. The most significant graphite characteristic that effects thebearing temperature is the heat transfer coefficient along the yarnparallel to the slip planes toward the heat sink. It is much higher inthis direction than laterally, if, in addition to the high densitypacking, the graphite crystals are oriented, particularly excellentbearing temperature characteristics are achieved at very high PV values.

Referring to FIG. 1 where there is shown a pair of graphite filaments inside by side relationship in a schematic bearing.

A species of the invention is to construct the bearing from graphitefilaments having oriented crystallites such as those illustrated-inFIG. 1. The filaments with oriented crystallites have expansionproperties which makes them particularly suitable for use as bearings.

Techniques for adjusting the coefficients of expansion of thegraphitefilaments are well-known, as the coefficient of expansion is afunction of the crystallite orientation. As the degree of crystalliteorientation increases, the ratio of lateral to longitudinal coefficientsof expansion can vary greatly, from as little as 3-l to as much as 50- ldepending upon the manufacturing process that orients the crystals inthe raw materials.

The ability to vary the ratio of lateral to longitudinal coefficients ofexpansion over such wide limits makes it possible to match thelongitudinal expansion of the filaments to the housing or rotating partor both if the housing and rotating part are formed from the samematerial. It is clear, therefore, that the very significant and criticalproblem associated with the misapplication of carbon-graphite bearingsin high temperature service due to its low coefficient of expansion iseliminated.

Because the oriented graphite has a higher coeffi-.

cient of thermal expansion in the lateral direction than in thelongitudinal direction, an end-loaded filament radial or thrust bearingof the type depicted in FIGS. 2 and 3 expand circumferentially.

The bearing surface tends to recede radially compensating for thermalexpansion of the shaft preventing the shaft from seizing in the bearing.The outer diameter expands into the bearing housing and typically, willgo into compression and become more firmly secured to the housing.Rotation of the bearing within its housing at high temperature is thusavoided.

Bearing materials having anisotropic thermal properties offer anotherbenefit. Heat conduction is greatest in the direction of crystalorientation. In the case of bearings, preferred crystal orientation isperpendicular to the bearing surface. Thus heat is preferentiallycarried away from the bearing surface. Bearing operating temperaturesare reduced.

Though the previous discussion has stressed endloaded carbonaceousfilaments for bearings, an alternative material having anisotropicthermal properties is pyrolytic graphite--see Pappas and Blum,Properties of Pyrolytic Graphite, Journal of The American CeramicSociety, Vol 44, No. 12, page 502.

A preferred way of making a radial bearing IS illustrated in FIGS. 4 and5. The symbol 30 identifies a right circular helix formed preferably ofwire or any other suitable material. The helix 30 is expanded so thatthere is a space between adjacent turns of the helix. A radial filamentstructure is constructed by passing a filament a diametrically acrossthe circle formed by one turn of the helix. A second filament b" is alsodiametrically placed across the turn but adjacent to filament a." Insequence, filaments c," d, e," etc., are likewise diametrically placedacross the turn as indicated until the entire surface formed by one turnof the helix is covered with diametrically placed filaments.

It is clear that the side by side filament relationship may be formed byindexing the helix 30 about its axis after each filament is placed or bymaintaining the helix fixed and moving the filaments around theperiphery of the helix. An over-view of a covered turn is illustrated inFIG. 5. Note that the diametrically placed filaments from a radialstructure. To form the bearing the helix is then compressed axiallyunder high pressure to form a filament mass having a density ir. excessof percent of the theoretical density of the filament material. A

shaft hole illustrated in phantom outline 31 is formed by conventionaldrilling and finishing techniques.

The various features and advantages of the invention are thought to beclear from the foregoing description. Various other features andadvantages not specifically enumerated will undoubtedly occur to thoseversed in the art, as likewise will many variations and modifications ofthe preferred embodiment illustrated, all of which may be achievedwithout departing from the spirit and scope of the invention as definedby the following claims:

We claim:

A method of making a radial bearing containing and loaded filamentscomprising the steps of:

a. expanding a wire helix axially;

b. positioning a first filament diametrically across the surface formedby one turn of said helix;

c. indexing said helix through a predetermined angular rotation;depositing another diametric filament next to said first filament;

e. repeat steps (0) and (d) until the entire surface is covered withfilaments;

f. compress the helix axially to form a plurality of contiguous surfaceswith filaments radially oriented; and

g. forming a passage through said surfaces.

1. A method of making a radial bearing containing and loaded filamentscomprising the steps of: a. expanding a wire helix axially; b.positioning a first filament diametrically across the surface formed byone turn of said helix; c. indexing said helix through a predeterminedangulAr rotation; d. depositing another diametric filament next to saidfirst filament; e. repeat steps (c) and (d) until the entire surface iscovered with filaments; f. compress the helix axially to form aplurality of contiguous surfaces with filaments radially oriented; andg. forming a passage through said surfaces.